\documentclass{article}
\usepackage{amssymb,amsmath,amscd,concmath}
\pagestyle{empty}
\begin{document}
\begin{equation*} X =
  \begin{cases}
     X & \mbox{if } X \ge 0 \\
     -X & \mbox{if } X < 0.
  \end{cases}
\end{equation*}
\end{document}
